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Modeling anaerobic digestion of aquatic plants by rumencultures: Cattail as an example

Bai-Hang Zhaoa, Zheng-Bo Yuea, Bing-Jie Nia, Yang Mua, Han-Qing Yua,*, Hideki Haradab

aDepartment of Chemistry, University of Science & Technology of China, Hefei, 230026 ChinabDepartment of Civil Engineering, Tohoku University, Sendai 980-8579, Japan

a r t i c l e i n f o

Article history:

Received 29 September 2008

Received in revised form

19 December 2008

Accepted 5 February 2009

Published online 14 February 2009

Keywords:

Anaerobic digestion

Aquatic plant

Cattail

Hydrolysis

Modeling

Rumen microbes

Lignocellulose

* Corresponding author. Tel.: þ86 551 360759E-mail address: hqyu@ustc.edu.cn (H.-Q.

0043-1354/$ – see front matter ª 2009 Elsevidoi:10.1016/j.watres.2009.02.006

a b s t r a c t

Despite of the significance of the anaerobic digestion of lignocellulosic materials, only

a limited number of studies have been carried out to evaluate the lignocellulosic digestion

kinetics, and information about the modeling of this process is limited. In this work,

a mathematical model, based on the Anaerobic Digestion Model No.1 (ADM1), was devel-

oped to describe the anaerobic conversion of lignocellulose-rich aquatic plants, with cattail

as an example, by rumen microbes. Cattail was fractionated into slowly hydrolysable

fraction (SHF), readily hydrolysable fraction (RHF) and inert fraction in the model. The SHF

was hydrolyzed by rumen microbes and resulted in the production of RHF. The SHF and

RHF had different hydrolysis rates but both with surface-limiting kinetics. The rumen

microbial population diversity, including the cattail-, butyrate-, acetate- and H2-degraders,

was all incorporated in the model structure. Experiments were carried out to identify the

parameters and to calibrate and validate this model. The simulation results match the

experimental data, implying that the fractionation of cattail into two biodegradation parts,

i.e., SHF and RHF, and modeling their hydrolysis rate with a surface-limiting kinetics were

appropriate. The model was capable of simulating the anaerobic biodegradation of cattail

by the rumen cultures.

ª 2009 Elsevier Ltd. All rights reserved.

1. Introduction (Ribes et al., 2004), it is found that this equation is unable to

The Anaerobic Digestion Model No.1 (ADM1) has been

proposed in early 2000s (Batstone et al., 2002), and has been

widely used to describe the anaerobic degradation of both

dissolved substrates and particulate, such as distillery

wastewater, dairy wastewater, cattle manure, primary sludge,

and municipal wastes (Tartakovsky et al., 2002; Flotats et al.,

2006; Vlyssides et al., 2007; Yasui et al., 2008). In ADM1, the

first-order kinetic equation is used to describe the hydrolysis

and microbial decay, and the Monod equation is used to

interpret the soluble components uptake and gas production.

Although the Monod equation is successfully used in

modeling the kinetics of cultures utilizing simple substrates

2; fax: þ86 551 3601592.Yu).er Ltd. All rights reserved

well predict the anaerobic hydrolysis of organic materials,

such as lignocellulosic wastes, because of their complexity in

composition and the multiple-hydrolysis process.

Recently, anaerobic conversion of lignocellulosic materials

is attracting increasing interests, attributed to the energy

crisis and their huge amount of annual production (Macias-

Corral et al., 2008). Lignocellulosic materials are usually the

main component of several types of agricultural, municipal,

and industrial wastes, e.g., corn stover, waste wood, and

paper. They can also be produced from the growth and

harvest of aquatic plants. Aquatic plant has been increasingly

used in phytoremediation for polluted water and contami-

nated soil (Yavitt and Knapp, 1998; DeLaune et al., 1999;

.

Table 1 – Composition of cattail.

Item Percentage Removal efficiency (%)

Total solids (TS) (%) 90.2� 1.3 65.4� 2.3

Volatile solids (VS) (%TS) 91.2� 2.3 67.6� 2.7

Neutral detergent

fiber (%TS)

64.1� 6.2 64.2� 1.8

Acid detergent 41.5� 5.3 54.5� 1.7

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 2 0 4 7 – 2 0 5 52048

Gajalakshmi et al., 2001; Bankston et al., 2002). However, the

disposal of the large amount of aquatic plant produced in the

phytoremediation process becomes a serious problem. This

consequently limits the wide application of aquatic plants in

pollution abatement (Gajalakshmi et al., 2001). Aquatic plant

is mainly composed of lignocellulose and has a great potential

to supply with energy resources of a variety of fermentation

products (Hu et al., 2007). The anaerobic conversion efficiency

of lignocellulosic materials is low in bioreactors seeded with

conventional anaerobic sludge, attributed to the low cellulo-

lytic activity and specific growth rate of the anaerobic micro-

organisms (Barnes and Keller, 2003). Rumen microbes have

been demonstrated to significantly increase the anaerobic

conversion rate of cattail in our recent studies (Hu et al., 2006,

2007), due to the complete enzyme components and high

enzyme activities (Hu et al., 2007).

In the degradation of lignocellulosic materials, rumen

microorganisms first attach to their surface, and then secrete

enzymes to digest the components (Russell and Wilson, 1996).

The rate of hydrolysis by these organisms is dependent on the

surface area of the particles occupied by the organisms (Pav-

lostathis and Giraldo-Gomez, 1991). This suggests the first-

order kinetic equation is not appropriate to describe the

hydrolysis process, which is described by ADM1. Thus, the

hydrolysis should be considered as a surface-limiting step

(Munch et al., 1999; Ruel et al., 2002; Dimock and Morgenroth,

2006). When the particle surfaces are fully occupied with

microorganisms, the rate will be first-order with respect to

particulate concentration; when the surfaces are not fully

occupied at a low biomass concentration, the hydrolysis rate

will be first-order with respect to biomass concentration

(Myint et al., 2007). Despite of the significance of the anaerobic

digestion of lignocellulosic wastes, only a limited number of

studies have been carried out to evaluate the lignocellulosic

digestion kinetics (Hu et al., 2007), and few information about

the modeling of anaerobic degradation of lignocellulosic

wastes by rumen cultures can been found in literatures.

This work aimed to establish a mathematical model with

an emphasis on anaerobic hydrolysis, and use the modified

kinetic model to describe the anaerobic degradation of ligno-

cellulose-rich aquatic plants by anaerobes, with the anaerobic

conversion of cattail (Typha latifolia linn) by rumen microbes

as an example. Cattail can utilize solar energy effectively and

grow rapidly, and thus has been widely utilized in phytor-

emediation for polluted water and contaminated soil

(Hu et al., 2006). In our new model, cattail was appropriately

fractionated and the hydrolysis kinetics was modified as the

surface-limiting kinetics. The model was then calibrated and

validated using our experimental results. It is expected that

this model is useful to better understand the behavior of

anaerobic degradation of aquatic plants by rumen microbes.

fiber (%TS)

Hemicellulose (%TS) 22.6� 2.5 68.7� 2.3

Cellulose (%TS) 20.8� 2.8 59.0� 0.9

Lignin (%TS) 10.5� 1.4 23.9� 1.3

Ash (%TS) 8.8� 0.3 –

Hemicellulose and cellulose as SDF, lignin and ash as inert fraction,

the remaining part in cattail as RDF.

Note: Standard deviations were calculated from the three

measurements.

2. Materials and methods

2.1. Seed microorganisms and substrate

Rumen fluid, obtained from a fistulated goat, was strained

through a four-layer gauze as inoculum. The media used in

the batch experiments contained the following ingredients

(in g L�1): NaHCO3 8.0; KH2PO4 1; K2HPO4 3.0; CaCl2$2H2O 0.03;

MgCl2$6H2O 0.08; NH4Cl 0.18; L-cysteine-HCl 0.173; and 1 ml

modified Pfennig’s metal solution, which had the following

composition (in g L�1): ZnSO4$7H2O 0.1; MnCl2$4H2O 0.03;

H3BO3 0.3; CoCl2$6H2O 0.2; CuCl2$2H2O 0.01; NiCl2$6H2O 0.02;

NaMoO4$2H2O 0.03; FeCl2$4H2O 1.5.

Cattail was obtained from a river in Hefei, China. The sun-

dried cattail was milled into powders with an average diam-

eter of 0.42 mm and specific surface of 14.3 mm�1. The

powders were then used as the substrate for fermentation.

The composition of cattail is shown in Table 1.

2.2. Experimental setup

Rumen microorganisms were grown with cattail as the sole

carbon and energy source. Inoculums were cultured in a 5.0 L

continuous-flow stirred tank reactor (CSTR) with a 2.5 L

working volume. The strained rumen fluid of 200 ml was used

as the inoculum. The temperature was maintained at

39� 1 �C, and the pH was controlled at predetermined values

6.9� 0.1 by the automatic addition of 4 N HCl or 4 N NaOH.

Agitation was kept constant at 120 rpm.

2.3. Analytical methods

Volatile fatty acids (VFAs) were determined using a gas chro-

matograph (GC-6890N, Agilent Inc., USA) equipped with

a flame ionization detector and a 30 m� 0.25 mm� 0.25 mm

fused-silica capillary column (DB-FFAP). The temperatures of

the injector and detector were 250 �C and 300 �C, respectively.

The oven temperature was initially at 70 �C for 3 min, followed

by a ramp-up of 20 �C/min for 5.5 min and held at a final

temperature of 180 �C for 3 min. Nitrogen was used as the

carrier gas with a flow rate of 2.6 mL/min. Neutral detergent

fiber, acid detergent fiber, cellulose, hemicellulose, lignin and

ash contents of cattail were measured according to Goering

and van Soest (1970). The analytical procedures are given in

Fig. 1. Measurements of chemical oxygen demand (COD), total

solids (TS) and volatile solids (VS) were analyzed according to

the standard methods (APHA/AWWA/WEF, 1995). Standard

deviations were calculated with the three measurements.

Cellulose, Hemicellulose, Lignin and Ash(NDF)

Cell contents

Cellulose, Lignin and Ash(ADF) Hemicellulose

Cattail

Lignin and AshCellulose

AshLignin

Digest with neutral detergent

Digest with acid detergent

Digest with 72 sulfuric acid

Remained at 650°C for 4 h

Fig. 1 – Analytical procedures for cattail.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 2 0 4 7 – 2 0 5 5 2049

Biogas was measured using the water displacement

method. Biogas was analyzed by another gas chromatograph

(SP-6800, Lunan Co., China) equipped with a thermal

conductivity detector and a 1.5 m� 2 mm stainless-steel

column packed with 5 A molecular sieves. The temperatures

of the injector, detector and column were kept at 100, 105 and

60 �C, respectively. Argon was used as the carrier gas at a flow

rate of 30 mL/min.

3. Model development and simulation

This kinetic model describes the relationships among three

species: substrate, products and microorganisms; eleven bio-

processes; and four kinds of microbes. In addition to the

Fig. 2 – COD flows in the kinetic model: (A) hydrolysis

cattail-degraders, which were responsible for the hydrolysis

of solid particulates and consumption of soluble sugars in the

degradation of cattail, other microorganisms were also

present in the reactor for the uptake of VFAs and hydrogen.

Except CO2, the units for all species, such as solid substrate,

various microbes, soluble substances, and gases, in the model

are kgCOD/m3, which is directly proportional to electron

equivalents (8 g O2 per e- equivalent) (Batstone et al., 2002).

The unit of CO2 is kmol/m3.

Hydrolysis is usually the rate-limiting step in the anaerobic

degradation of solid wastes (Pavlostathis and Giraldo-Gomez,

1991). A higher hydrolysis rate results in a higher production

rate of both intermediate and end products. The end-product

formation is greatly related to the characteristics of the

primary substrate. For instance, Yasui et al. (2008) divided the

primary sludge into three biodegradation parts according to

their methane production rate curves. Several studies on the

anaerobic digestion of aquatic plants (e.g., cattail) by rumen

microbes show that the VFA formation in such a system has

two distinct regions: a relatively faster initial fermentation,

where easily digestible portions of the substrate are con-

verted, followed by a slower fermentation, where the more

refractory portions of the substrate are consumed (Hu and Yu,

2006; Hu et al., 2006; Yue et al., 2007). Thus, it is reasonable to

assume that cattail was divided into two biodegradation parts

(SDF and RDF) and one inert part.

All of the COD flows of the proposed model are sche-

matically illustrated in Fig. 2. The substrates utilized were

either diverted into cellular materials or converted to

various products. The pathways from solid substrate to

soluble fraction are shown in Fig. 2a. Based on the charac-

teristics of cattail, the solid substrate was disintegrated into

three parts, slowly hydrolysable fraction (SHF), readily

hydrolysable fraction (RHF) and inert fraction (process 1).

Hypothetically, the SHF hydrolyzed, and generated the RHF

and the inert solid under the activity of cattail-degraders

(process 2). Thereafter, the fraction of RHF was hydrolyzed

and produced soluble sugars, which were the actual

substrate utilized by the cattail-degraders, as well as soluble

inert fraction (process 3).

; and (B) acidification and methanation processes.

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w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 2 0 4 7 – 2 0 5 52050

Fig. 2b shows the relationships of the various soluble frac-

tions and relevant microorganisms, the approach of substrate

transformation, and the microbial growth and decay. The

soluble component and the gas were removed by relevant

microorganisms in the processes 4–7. The growth of four types

of microorganisms was coupling with the substrates uptake in

the processes 4–7. The processes 8–11 represented the decay of

microorganisms, i.e., the cattail-, butyrate-, acetate-, and

H2-degraders. The microbial decay in the processes 8–11

released inert residual particulate compounds, which didn’t

participate in the biochemical reactions.

Related process kinetics and stoichiometry are presented in

a matrix format in Table 2 to highlight the interactions among

the model components and processes. All the parameters

(with a 95% ofconfidence interval)usedinthe model developed

in this work, their symbols and units are defined in Table 3.

A computer program, AQUASIM 2.0 (Reichert, 1998), is used

for modeling the biological processes. AQUASIM 2.0 is

a program designed mainly for estimating the coefficients and

parameters involved in a bioreaction model. This program

offers flexible definition of the kinetic model, flow scheme, and

process control strategies; it also provides support for graphic

display of the support of the simulation results, corresponding

experimental data, and communication with spreadsheet

programs (Reichert, 1998).

4. Results and discussion

4.1. Reactor performance

In this study, the removal efficiency of VS by the rumen

microbes reached about 70%, which is higher than that with

the anaerobic sludge from usual sources (Camp et al., 1989).

The high VS removal efficiency by the rumen biomass has

also been reported by Barnes and Keller (2003) and Yue et al.

(2007). In anaerobic digestion of cattail, the main aqueous

intermediates were acetate, propionate, and butyrate.

4.2. Fractionation of cattail for biodegradation

In our model, cattail was divided into three parts as SHF, RHF

and inert fraction. The cellulose and hemicellulose are usually

considered together as a single substrate (Vavilin et al., 2002).

Similarly, the cellulose and hemicellulose were considered as

a single substrate in our model, i.e., SHF. The carbohydrates in

cell contents were regarded as RHF and the others in cattail

were considered as inert solid fraction. The fractions of both

crude proteins and fat in cattail were small and thus were not

taken into account, in order to simplify the model structure.

As shown in Table 1, the lignin component in cattail was

10.5%, which only can be degraded by 23.9%. The macromo-

lecular properties and structural characteristics of lignin

make its biodegradation difficult (Tuomela et al., 2002), which

results in a very low lignin biodegradable. Since the degraded

lignin was only about 2% of TS in cattail, lignin was regarded

as a part of inert solid substrate in our kinetic model. The

above stoichiometric parameters are summarized in Table 3.

Table 3 – Stoichiometric and kinetic parameters in the established model.

Symbol Definition Unit Value Source

Stoichiometrics

fSF,C fraction of SHF in substrate kgCOD/kgCOD 0.55� 0.09 calculated

fRF,C fraction of RHF in substrate kgCOD/kgCOD 0.25� 0.07 calculated

fi,C fraction of inertia in substrate kgCOD/kgCOD 0.20� 0.02 calculated

fRF,SF fraction of RHF from SHF hydrolysis kgCOD/kgCOD 0.70� 0.11 calculated

fi,SF fraction of inertia from SHF hydrolysis kgCOD/kgCOD 0.30� 0.09 calculated

fsu,RF fraction of inertia from RHF hydrolysis kgCOD/kgCOD 0.95� 0.04 calculated

fi,RF fraction of inertia from RHF hydrolysis kgCOD/kgCOD 0.05� 0.01 calculated

fbu,su fraction of butyrate from sugar degradation kgCOD/kgCOD 0.13 default value

fpr,su fraction of propionate from sugar degradation kgCOD/kgCOD 0.27 default value

fac,su fraction of acetate from sugar degradation kgCOD/kgCOD 0.41 default value

fH2 ;su fraction of hydrogen from sugar degradation kgCOD/kgCOD 0.19 default value

Yca Cattail-degraders yield kgCOD/kgCOD 0.10 default value

Ybu butyrate degraders yield kgCOD/kgCOD 0.06 default value

Yac acetate degraders yield kgCOD/kgCOD 0.05 default value

YH2 H2-degraders yield kgCOD/kgCOD 0.06 default value

Kinetic parameters

kdis Disintegration rate coefficient d�1 0.4� 0.1 estimated

khyd,SF hydrolysis rate constant for SHF d�1 2.41� 0.12 estimated

khyd,RF hydrolysis rate constant for RHF d�1 9.68� 2.41 estimated

km,su Maximum specific uptake rate for sugar d�1 30 default value

km,bu Maximum specific butyrate- uptake rate d�1 20 default value

km,ac Maximum specific acetate-uptake rate d�1 21 default value

km;H2 Maximum specific H2-uptake rate d�1 35 default value

Khyd,SF Hydrolysis saturation constant for SHF kgCOD/m3 0.05� 0.09 estimated

Khyd,RF Hydrolysis saturation constant for RHF kgCOD/m3 0.05� 0.12 estimated

Ks,su Half-saturation coefficient for sugars kgCOD/m3 2.48� 3.79 estimated

Ks,bu Half-saturation coefficient for butyrate kgCOD/m3 0.045� 0.078 estimated

Ks,ac Half-saturation coefficient for acetate kgCOD/m3 0.518� 0.531 estimated

Ks;H2 Half-saturation coefficient for H2 kgCOD/m3 0.130� 0.345 estimated

kdec,ca Decay coefficient for cattail-degraders d�1 0.02 default value

kdec,bu Decay coefficient for butyrate-degraders d�1 0.02 default value

kdec,ac Decay coefficient for acetate-degraders d�1 0.02 default value

kdec;H2Decay coefficient for H2-degraders d�1 0.02 default value

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 2 0 4 7 – 2 0 5 5 2051

4.3. Sensitivity analysis

The hydrolysis is the rate-limiting step in the anaerobic

digestion of cattail by rumen microorganisms. A sensitivity

analysis was performed to identify the most sensitive

parameters in the two-step hydrolysis processes. For the

parameters, the dimensionless values are obtained by

dividing their nominal values (reference values) by the arbi-

trary value with a �60% change in the nominal value. For the

outputs VS, total volatile fatty acid (TVFA) and CH4, their

dimensionless values are obtained by dividing the basic

values (reference values) by the arbitrary value with a �60%

change in the basic values. The basic values for the outputs

VS, TVFA and CH4 were obtained from model calculation

through inputting the nominal parameters values. The refer-

ence values are listed in Table 3. The model parameters are

independent, and they were investigated individually. The

four parameters in the hydrolysis processes were changed one

by one in the simulation. Parameters were assigned with an

initial value shown in Table 3 for calibration.

The results of sensitivity analysis are shown in Fig. 5. The

output results of VS, TVFA and CH4 became almost unchanged

when dimensionless khyd,SF> 0.9, implying that these

concentrations reached asymptotic values, which essentially

remained constant even with a further increase in dimen-

sionless khyd,SF. It is found that a 60% increase in khyd,SF will

result in 230% decrease in the VS, 40% increase in TVFA and

18% increase in CH4. Fig. 5B shows that �60% fluctuation in

Khyd,SF will result in �0.3% fluctuation in VS, �0.5 fluctuation

in TVFA and �0.02% fluctuation in CH4. Similarly, �60% fluc-

tuation in khyd,RF will result in �0.2% fluctuation in VS, �0.4%

fluctuation in TVFA and �0.05% fluctuation in CH4 (Fig. 5C);

�60% fluctuation in Khyd,RF will result in �0.07% fluctuation in

VS, �0.06% fluctuation in TVFA and �0.02% fluctuation in CH4

(Fig. 5D).

The results above indicate that the hydrolysis rate

constant khyd,SF for SHF was most sensitive, and the sensi-

tivity of other parameters followed the order: Khyd,SF> -

khyd,RF> Khyd,RF. It also suggests that the SHF hydrolysis was

the predominated step in cattail degradation. This result is in

agreement with the previous reports in which hydrolysis was

found to be the rate-limiting step in the anaerobic digestion

of particulate substrates (Veeken et al., 2000; Vavilin et al.,

2002; Higuchi et al., 2005; Myint et al., 2007). The sensitivity

analytical results provide useful information for the subse-

quent calibration.

0 2 4 6 8 100.0

0.7

1.4

2.1

2.8

0 2 4 6 8 100.0

0.6

1.2

1.8

2.4

Acetate

Propionate

Butyrate

VF

A (

kg C

OD

/m3 )

CH

4 (k

g C

OD

/m3 )

Operating time (d)

A

B

Fig. 3 – Simulationvalues (line) andmeasuredvalues (dot) for

model calibration at VS of 10 kgCOD/m3: (A) VFA; and (B) CH4.

0 5 10 15 200.0

1.2

2.4

3.6

4.8

0 5 10 15 200.0

1.6

3.2

4.8

6.4

CH

4(kg

CO

D/m

3 )

Operating time (d)

B

Acetate

Propionate

Butyrate

VF

A (

kg C

OD

/m3 )

A

Fig. 4 – Simulationvalues (line) and measured values (dot) for

model validation at VS of 22 kgCOD/m3: (A) VFA; and (B) CH4.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 2 0 4 7 – 2 0 5 52052

4.4. Model calibration

The kinetic parameters in several new incorporated biological

processes in our model could not be found in literatures.

Therefore, the kinetic parameters for the hydrolysis must be

measured or estimated. The hydrolysis rate constants and

hydrolysis saturation constants were determined by fitting

the predicted VFA and CH4 values to the experimentally

measured data. In case of mismatch, the procedure was

repeated until the predicted results matched well with the

measured ones. A set of data for calibration is shown in Fig. 3

and a good agreement between the measured and predicted

results was achieved. As a result, the parameters of the

hydrolysis process for SHF and RHF were obtained and are

listed in Table 3.

The hydrolysis rate constants, khyd,SF and khyd,RF estab-

lished in our study for the surface-limiting model (2.41 d�1 for

SHF and 9.68 d�1 for RHF) are greater than the values reported

by Myint et al. (2007) for hemicellulose (1.4 d�1) and for

cellulose (0.09 d�1) seeded with usual anaerobic sludge. Values

(0.75 d�1 and 2.50 d�1) reported by Munch et al. (1999) and Ruel

et al. (2002) for complex wastes are also smaller than our

values. This suggests that the rumen microbes have a higher

capability to hydrolyze lignocellulosic wastes than the usual

anaerobic microbes.

The sensitivity analysis shows that even when the values

of Khyd,SF and Khyd,RF were changed by a factor of two, the

variations in the predicted VFA and CH4 values were less than

10% of the measured values. Thus, the hydrolysis saturation

constants for SHF and RHF were both set at 0.05 kgCOD/L in

our model. The values of other stoichiometric parameters and

kinetic parameters in our model are listed in Table 3. As

shown in Table 3, there are thirty-two stoichiometric and

kinetic parameters in the model. Among them, seven stoi-

chiometric parameters were calculated from the cattail

components and their removal efficiencies (Table 1); four

parameters in hydrolysis were estimated after sensitivity

analysis, because the hydrolysis process was crucial in the

model; sixteen parameters were the default values adopted

from ADM1; four half-saturation coefficients and one disin-

tegration rate coefficient were not the default values recom-

mended in ADM1, but were estimated from our experimental

data because of the differences in microbial population.

4.5. Model validation

The model verification is based on a comparison between the

experimental results and the calibrated model predictions

with the same input model parameters. The experimental

data, different from that used for model calibration, were

employed for model validation. As shown in Fig. 4, the simu-

lating results reasonably match the temporal trends of the

measured data with errors less than 10%. Such an agreement

validates the modeling approach as well as the model

parameters used in this study.

In addition to the two representative batch experimental

results in Figs. 3 and 4, the experimental results of Yue et al.

(2008) were also used for model simulation to validate the

established model in this work. Yue et al. (2008) carried out an

0.9990

0.9995

1.0000

1.0005

1.0010

0.998

1.000

1.002

1.004

0.4 0.8 1.2 1.60.4 0.8 1.2 1.60.996

0.998

1.000

1.002

1.004

0.00.4 0.8 1.2 1.6

0.9960.4 0.8 1.2 1.6

0.6

1.2

1.8

2.4

Dimensionless Khyd,SD

Dim

ensi

onle

ss y

Dim

ensi

onle

ss y

Dimensionless khyd,SD

Dimensionless Khyd,RDDimensionless khyd,RD

CH4

TVFA

VS

A B

C D

Fig. 5 – Effect of the parameter values involved in hydrolysis on output variables (-, y [ CH4; B, y [ TVFA; ,, y [ VS):

(A) khyd,SF; (B) Khyd,SF; (C) khyd,RF; and (D) Khyd,RF.

0 5 10 150.0

0.5

1.0

1.5

2.0

Con

cent

rati

on (

kg C

OD

/m3 )

Operating time (d)

VFA

CH4

Fig. 6 – The measured and simulated VFA and CH4 profiles

in the study of Yue et al. (2008).

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 2 0 4 7 – 2 0 5 5 2053

investigation into the anaerobic digestion of the bulrush pre-

treated with steam explosion. The measured and simulated

results of VFA and CH4 production profiles in this investiga-

tion are shown in Fig. 6. A good agreement between the

measured and simulated results was obtained. Such a simu-

lation clearly demonstrates that the model established in the

present work is also able to appropriately simulate the

anaerobic digestion of aqueous plants, regardless the

substrate type and operating conditions. This result also

further confirms the validation of the established model in our

work.

The first-order kinetics has been conventionally used to

describe the hydrolysis process in anaerobic digestion of

simple substrates, such as carbohydrates and proteins (Bat-

stone et al., 2002; Vlyssides et al., 2007). However, it is not able

to appropriately describe the hydrolysis of complex substrates

(Dimock and Morgenroth, 2006; Vavilin et al., 2008). Thus, in

the present work the ADM1 should been modified to more

accurately describe the anaerobic digestion of complex

substrates, e.g., aquatic plants. Based on analysis on the

anaerobic digestion of complex substrates by rumen microbes

(Russell and Wilson, 1996), a surface-limiting kinetics was

adopted and integrated into the modified ADM1 to describe

the hydrolysis process in this study. The simulation results

match the experimental data, implying that the hydrolysis of

both SHF and RHF could be described as a surface-limiting

kinetics.

As shown in Figs. 3B and 4B, a two-stage production of CH4

was found in the anaerobic biodegradation of cattail by rumen

cultures. A similar phenomenon has also been observed for

VFA production in the anaerobic fermentation reactors

(Hu and Yu, 2006; Hu et al., 2006). The end products were

formed after a series of processes and hence their formation

could reflect the characteristics of the initial substrate. The

initial CH4 production stage in the marked region of Figs. 3B

and 4B was attributed to the RHF hydrolysis, while the latter

production stage was mainly associated with the SHF hydro-

lysis. The methane production between 0.8 d and 3.8 d in

Fig. 4B was likely to be attributed to the soluble substrates

from seeding liquid.

The findings in this study regarding SDF and RDF can be of

values in designing, monitoring, analyzing, and optimizing

the anaerobic digestion process for the treatment of complex

substrates such aqueous plants. The established model also

provides an insight into the anaerobic digestion of aquatic

plants by rumen microbes in continuous-flow reactors,

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 2 0 4 7 – 2 0 5 52054

because of the consistent biochemical processes and kinetics

between batch and continuous experiments.

5. Conclusions

A mathematical model was developed to describe the anaer-

obic digestion of aquatic plants by rumen microbes in this

paper. The following conclusions are drawn:

� Cattail could be fractionated into slowly hydrolysable frac-

tion, readily hydrolysable fraction and inert fraction. With

such a fractionation, the different hydrolysis rates of the

two main degradable components in cattail could be

described as a surface-limiting kinetics well.

� The established model was found to be capable of simu-

lating and predicting the anaerobic degradation of cattail by

rumen microbes in terms of cattail degradation, microbial

growth, as well as formation of aqueous and gaseous

products under different conditions.

� The SHF hydrolysis was the rate-limiting step in the

anaerobic cattail degradation, and the hydrolysis rate

constant khyd,SF for SHF was the most significant parameter

in such a process. The sensitivity of other parameters to the

output of VS, TVFA and CH4 followed the order of

Khyd,SF> khyd,RF> Khyd,RF.

� This model is useful to better understand the behavior of

anaerobic degradation of aquatic plants by rumen microbes.

Acknowledgments

The authors wish to thank the NSFC-JST Joint Project

(20610002), the Chinese Academy of Sciences (KSCX2-YW-

G-001), National Basic Research Program of China

(2004CB719703), and National 863 Program of China

(2006AA06Z340), for the partial support of this study.

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